Propagate strict overrides through nested models

[kstrauser]

This allows model-instantion-type overrides of strictness settings so that use cases like this work:

    class Inner(Model):
        expected_key = StringType()

    class Outer(Model):
        """docstring"""
        inner = ModelType(Inner)

    doc = {
        'inner': {
            'expected_key': 'expected value',
            'rogue_key': 'unexpected value',
        },
    }

   print Outer(doc, strict=False).inner.expected_key

The default behavior of honoring the models' configured strictness is maintained, but explicitly setting strict=[value] overrides that behavior in expected ways.

[bintoro]

#359 fixed the propagation issue, but it tentatively removed the ModelType.strict option since I wasn't sure if there's an actual use case for it or if it was introduced as a half-baked way to get some control over the strictness of embedded models (since the propagation didn't work).

If there is indeed a true need for ModelType.strict, then let's leave this PR open until those tests pass too.

[kstrauser]

Yay! \o/